Can we be sure IPCC report hasn’t missed any big issues?

Not entirely. The IPCC scientists admit that some potential positive feedbacks from climate change have not been included in their model assessments of future temperatures. This is because they cannot yet be quantified sufficiently well.

For example, there is irritation that the report has sidelined the fear that methane, a potent greenhouse gas, may be released into the air in large quantities from melting permafrost and warming ocean beds. This would accelerate warming.

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As a result the new report is open to the charge that its climate projections are too conservative. Kevin Schaefer at the University of Colorado at Boulder of a report on the topic published last year for the UN Environment Programme, says that “by leaving out the permafrost feedback, all climate projections in the [new assessment] are likely to be biased on the low side.”

The model scenarios also don’t include the possible rapid decay of the Greenland and West Antarctic ice sheets, or the possibility of the collapse of the global ocean circulation system, again because the risks are hard to assess properly.

This disturbs some climate scientists. Michael Mann of Penn State University says&colon; “The report should not be dismissing impacts with lower probability, but higher threat potential.” It is critical for policymakers to know about these, he says.

The IPCC has been here before. There were complaints in 2007 when it decided to downplay concerns that the big ice sheets on Greenland and Antarctica could be disintegrating physically, something that could dump their contents into the oceans much more quickly than gradual melting. Those critics were proved right, and the new report almost doubles projections of sea level rise as a result.

Myles Allen of Oxford University, and IPCC lead author, says it is inevitable that the IPCC should be conservative. “It happened last time over sea-level rise. There may well be things this time that people will later say we were too conservative about. We never say we are 100 per cent certain the real world will lie within the range of the models.”